Intravascular Stent

ABSTRACT

The invention is directed to an expandable stent for implanting in a body lumen, such as a coronary artery, peripheral artery, or other body lumen. The invention provides for an intravascular stent having a plurality of cylindrical rings connected by undulating links. The stent has a high degree of flexibility in the longitudinal direction, yet has adequate vessel wall coverage and radial strength sufficient to hold open an artery or other body lumen. The stent can be compressed or crimped onto a catheter to a very low profile since the peaks that are adjacent the curved portion of the undulating link are shorter than other peaks in the same cylindrical ring to prevent overlap yet still achieve a very low profile, tightly crimped stent onto a catheter.

CROSS-REFERENCES TO RELATED APPLICATIONS

This is a divisional application of U.S. Ser. No. 12/416,753, filed Apr.2, 2009, which is a divisional application of U.S. Ser. No. 11/217,213,filed Aug. 31, 2005; which is a continuation application of U.S. Ser.No. 10/695,290 filed Oct. 27, 2003 which issued as U.S. Pat. No.6,939,373 on Sep. 6, 2005, which is a continuation of U.S. Ser. No.10/645,265 filed Aug. 20, 2003 which issued as U.S. Pat. No. 6,929,657on Aug. 16, 2005, which is a continuation of U.S. Ser. No. 09/879,263filed Jun. 11, 2001 now U.S. Pat. No. 6,629,994 issued Oct. 7, 2003.Applicant claims priority to each application in the chain. Each of theforegoing applications is incorporated herein by reference thereto.

BACKGROUND OF THE INVENTION

The invention relates to vascular repair devices, and in particularintravascular stents, which are adapted to be implanted into a patient'sbody lumen, such as a blood vessel or coronary artery, to maintain thepatency thereof. Stents are particularly useful in the treatment ofatherosclerotic stenosis in arteries and blood vessels.

Stents are generally tubular-shaped devices which function to hold opena segment of a blood vessel or other body lumen such as a coronaryartery. They also are suitable for use to support and hold back adissected arterial lining that can occlude the fluid passageway. Atpresent, there are numerous commercial stents being marketed throughoutthe world. For example, the prior art stents depicted in FIGS. 1-5 havemultiplex cylindrical rings connected by one or more undulating links.While some of these stents are flexible and have the appropriate radialrigidity needed to hold open a vessel or artery, there typically is atradeoff between flexibility and radial strength and the ability totightly compress or crimp the stent onto a catheter so that it does notmove relative to the catheter or dislodge prematurely prior tocontrolled implantation in a vessel.

What has been needed and heretofore unavailable is a stent which has ahigh degree of flexibility so that it can be advanced through tortuouspassageways and can be readily expanded, and yet have the mechanicalstrength to hold open the body lumen or artery into which it isimplanted and provide adequate vessel wall coverage. The presentinvention satisfies this need. That is, the stent of the presentinvention has a high degree of compressibility to secure it on thecatheter and provide a low profile and a high degree of flexibilitymaking it possible to advance the stent easily through tortuousarteries, yet the stent has sufficient radial rigidity so that it canhold open an artery or other blood vessel, or tack up a dissected liningand provide adequate vessel wall coverage.

SUMMARY OF THE INVENTION

The present invention is directed to an intravascular stent that has apattern or configuration that permits the stent to be tightly compressedor crimped onto a catheter to provide an extremely low profile and toprevent relative movement between the stent and the catheter. The stentalso is highly flexible along its longitudinal axis to facilitatedelivery through tortuous body lumens, but which is stiff and stableenough radially in its expanded condition to maintain the patency of abody lumen such as an artery when the stent is implanted therein.

The stent of the present invention generally includes a plurality ofcylindrical rings that are interconnected to form the stent. The stenttypically is mounted on a balloon catheter if it is balloon expandableor mounted on or in a catheter without a balloon if it isself-expanding.

Each of the cylindrical rings making up the stent have a proximal endand a distal end and a cylindrical plane defined by a cylindrical outerwall surface that extends circumferentially between the proximal end andthe distal end of the cylindrical ring. Generally the cylindrical ringshave a serpentine or undulating shape which includes at least oneU-shaped element, and typically each ring has more than one U-shapedelement. The cylindrical rings are interconnected by at least oneundulating link which attaches one cylindrical ring to an adjacentcylindrical ring. The undulating links are highly flexible and allow thestent to be highly flexible along its longitudinal axis. At least someof the undulating links have a curved portion that extends transverse tothe stent longitudinal axis for a predetermined distance that coincideswith one of the U-shaped elements. More specifically, the curved portionextends in a transverse manner such that it would intersect with thecorresponding U-shaped element, however, the corresponding U-shapedelement is shorter in length than other U-shaped elements in the samering. Thus, when the stent is compressed or crimped onto the catheter,the curved portions do not overlap or intersect with the adjacentU-shaped element since that element is shorter in length than similarU-shaped elements in the particular ring. In this manner, the stent canbe compressed or crimped to a much tighter or smaller diameter onto thecatheter which permits low profile delivery as well as a tight grippingforce on the catheter to reduce the likelihood of movement between thestent and the catheter during delivery and prior to implanting the stentin the vessel.

The undulating links may take various configurations but in general havean undulating or serpentine shape. The undulating links can includebends connected by substantially straight portions wherein thesubstantially straight portions are substantially perpendicular to thestent longitudinal axis.

Not only do the undulating links that interconnect the cylindrical ringsprovide flexibility to the stent, but the positioning of the links alsoenhances the flexibility by allowing uniform flexibility when the stentis bent in any direction along its longitudinal axis. Uniformflexibility along the stent derives in part from the links of one ringbeing circumferentially offset from the links in an adjacent ring.Further, the cylindrical rings are configured to provide flexibility tothe stent in that portions of the rings can flex or bend and tipoutwardly as the stent is delivered through a tortuous vessel.

The cylindrical rings typically are formed of a plurality of peaks andvalleys, where the valleys of one cylindrical ring are circumferentiallyoffset from the valleys of an adjacent cylindrical ring. In thisconfiguration, at least one undulating link attaches each cylindricalring to an adjacent cylindrical ring so that at least a portion of theundulating links is positioned within one of the valleys and it attachesthe valley to an adjacent peak.

While the cylindrical rings and undulating links generally are notseparate structures, they have been conveniently referred to as ringsand links for ease of identification. Further, the cylindrical rings canbe thought of as comprising a series of U's, W's and Y-shaped structuresin a repeating pattern. Again, while the cylindrical rings are notdivided up or segmented into U's, W's and Y's, the pattern of thecylindrical rings resemble such configuration. The U's, W's and Y'spromote flexibility in the stent primarily by flexing and by tippingradially outwardly as the stent is delivered through a tortuous vessel.

The undulating links are positioned so that the curved portion of thelink is outside the curved part of the W-shaped portion. Since thecurved portion does not substantially expand (if at all) when the stentis expanded, it will continue to provide good vessel wall coverage evenas the curved part of the W-shaped portion spreads apart as the stent isexpanded. The curved portion of the link extends in a directiontransverse to the stent longitudinal axis for a distance that positionsit adjacent and proximal to the peak of a U-shaped element. TheseU-shaped elements have struts that are shorter than the struts of theother U-shaped elements in the same cylindrical ring so that as thestent is compressed the curved portion of the link does not overlap theadjacent U-shaped element.

In one embodiment, the W-shaped portion has a first and second radius atits base where the first radius is greater than the second radius sothat the first radius expands more easily than the second radius whenthe stent is expanded. The first radius corresponds with a second peak(U-shaped member) which is shorter than the other peaks in the ring. Thesecond peak has shorter struts than the struts of the other peaks and asa result expands more slowly when the stent expands. Thus, fasterexpansion rate of the first radius of the W-shaped portion has atendency to compensate for the slower expansion rate of the adjacentshorter second peak to provide overall uniform expansion of the stent.Also, the shorter second peak can have a greater radius than the longerfirst peaks, again to provide different expansion rates to obtain moreuniform stent expansion.

In another embodiment, each ring has nine peaks, three each of first,second, and third peaks. The third peak has the longest struts, thesecond peak the shortest struts, and the first peak has intermediatelength struts. In order to obtain uniform stent expansion, the radius ofthe peaks is inversely proportional to the strut length. The shortersecond peak with the shortest struts has the biggest peak radius, thefirst peak has an intermediate radius, and the third peak with thelongest struts has the smallest peak radius.

The number and location of undulating links that interconnect adjacentcylindrical rings can be varied as the application requires. Since theundulating links typically do not expand when the cylindrical rings ofthe stent expand radially outwardly, the links are free to continue toprovide flexibility and to also provide a scaffolding function to assistin holding open the artery. Importantly, the addition or removal of theundulating links has very little impact on the overall longitudinalflexibility of the stent. Each undulating link is configured so that itpromotes flexibility whereas some prior art connectors actually reduceflexibility of the stent.

The cylindrical rings of the stent are plastically deformed whenexpanded when the stent is made from a metal that is balloon expandable.Typically, the balloon-expandable stent is made from a stainless steelalloy or similar material.

Similarly, the cylindrical rings of the stent expand radially outwardlywhen the stent is formed from superelastic alloys, such asnickel-titanium (NiTi) alloys. In the case of superelastic alloys, thestent expands upon application of a temperature change or when a stressis relieved, as in the case of a pseudoelastic phase change.

Because of the undulating configuration of the links, the stent has ahigh degree of flexibility along the stent axis, which reduces thetendency of stent fishscaling. Stent fishscaling can occur when thestent is bent and portions of the stent project outward when the stentis in the unexpanded condition. The present invention undulating linksreduce the likelihood of fishscaling.

Further, because of the positioning of the links, and the fact that thelinks do not expand or stretch when the stent is radially expanded, theoverall length of the stent is substantially the same in the unexpandedand expanded configurations. In other words, the stent will notsubstantially shorten upon expansion.

The stent may be formed from a tube by laser cutting the pattern ofcylindrical rings and undulating links in the tube. The stent also maybe formed by laser cutting a flat metal sheet in the pattern of thecylindrical rings and links, and then rolling the pattern into the shapeof the tubular stent and providing a longitudinal weld to form thestent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view, partially in section, of a prior artstent mounted on a rapid-exchange delivery catheter and positionedwithin an artery.

FIG. 2 is an elevational view, partially in section, similar to thatshown in FIG. 1 wherein the prior art stent is expanded within theartery, so that the stent embeds within the arterial wall.

FIG. 3 is an elevational view, partially in section, showing theexpanded prior art stent implanted within the artery after withdrawal ofthe rapid-exchange delivery catheter.

FIG. 4 is a plan view of a flattened prior art stent which illustratesthe pattern of the stent shown in FIGS. 1-3.

FIG. 5 is a side view of the prior art stent of FIG. 4 in a cylindricalconfiguration and in an unexpanded state.

FIG. 6A is a plan view of a flattened stent of one embodiment of theinvention which illustrates the pattern of the rings and links.

FIG. 6B is a partial plan view of the stent of FIG. 6A which has beenexpanded to approximately 3.0 mm inside diameter.

FIG. 6C is a plan view of a portion of the stent of FIG. 6A rolled intoa cylindrical configuration and tightly crimped so that the variousstent struts are either in close contact or contacting each other.

FIG. 7A is a plan view of a flattened stent of another embodiment of theinvention which illustrates the pattern of the rings and links.

FIG. 7B is a partial plan view of the stent of FIG. 7A which has beenexpanded to approximately 4.0 mm inside diameter.

FIG. 7C is a portion of the stent of FIG. 7A that is illustrated in acylindrical configuration and is tightly crimped or compressed.

FIG. 8A is a plan view of a flattened stent of another embodiment of theinvention which illustrates the pattern of the rings and links.

FIG. 8B is a plan view of the flattened stent of FIG. 8A where the ringsand links have, been crimped or tightly compressed.

FIG. 8C is a plan view of a portion of the flattened stent of FIG. 8Aillustrating the relationship of the U-shaped member to the undulatinglink prior to crimping the stent.

FIG. 9A is a plan view of a flattened stent of another embodiment of theinvention which illustrates the pattern of the rings and links.

FIG. 9B is a plan view of the flattened stent of FIG. 9A where the ringsand links have been crimped or tightly compressed.

FIG. 9C is a portion of the flattened stent of FIG. 9A illustrating therelationship of the shortened U-shaped member and the undulating portionof the link when the stent is in a partially crimped or compressedconfiguration.

FIG. 10A is a plan view of a flattened stent of another embodiment ofthe invention which illustrates the pattern of the rings and links.

FIG. 10B is a plan view of the flattened stent of FIG. 10A in a crimpedor compressed configuration.

FIG. 10C is a partial plan view of the flattened stent of FIG. 10Adepicting the relationship between the shortened U-shaped member and theundulating portion of the link when the stent is partially crimped orcompressed.

FIG. 11A is a plan view of a flattened stent of another embodiment ofthe invention which illustrates the pattern of the rings and links.

FIG. 11B is a plan view of the stent of FIG. 11A depicting the rings andlinks in a crimped or compressed configuration.

FIG. 11C is a partial plan view of the flattened stent of FIG. 11Adepicting the relationship between the shortened U-shaped member and theundulating portion of the link when the stent is partially linked orcompressed.

FIG. 12 is a plan view of the stent of FIG. 10A rolled into acylindrical configuration and in a crimped or compressed configuration.

FIG. 13 is a plan view of the stent of FIG. 10A in a cylindricalconfiguration and illustrating the rings and links in an expandedconfiguration.

FIG. 14 is a plan view of a flattened stent of another embodiment of theinvention which illustrates the pattern of rings and links.

FIG. 15 is a plan view of a flattened stent of another embodiment of theinvention which illustrates the pattern of the rings and links whereeach of the rings has nine peaks.

FIG. 16 is a plan view of a flattened stent of another embodiment of theinvention which illustrates the pattern of the rings and links.

FIG. 17 is a plan view of a flattened stent depicting another embodimentof the invention which illustrates the pattern of rings and links.

FIG. 18 is an enlarged partial perspective view of a portion of a peakand associated struts depicting variable thickness struts.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention stent improves on existing stents by providing alongitudinally flexible stent having a uniquely designed pattern andnovel interconnecting members. In addition to providing longitudinalflexibility, the stent of the present invention also provides radialrigidity and a high degree of scaffolding of a vessel wall, such as acoronary artery. The design of the highly flexible interconnectingmembers and their placement relative to an adjacent U-shaped memberprovides for a tightly compressed stent onto a catheter whilemaintaining a high degree of flexibility during delivery.

Turning to the drawings, FIG. 1 depicts a prior art stent 10 mounted ona conventional catheter assembly 12 which is used to deliver the stentand implant it in a body lumen, such as a coronary artery, peripheralartery, or other vessel or lumen within the body. The catheter assemblyincludes a catheter shaft 13 which has a proximal end 14 and a distalend 16. The catheter assembly is configured to advance through thepatient's vascular system by advancing over a guide wire by any of thewell known methods of an over the wire system (not shown) or a wellknown rapid exchange catheter system, such as the one shown in FIG. 1.

Catheter assembly 12 as depicted in FIG. 1 is of the well known rapidexchange type which includes an RX port 20 where the guide wire 18 willexit the catheter. The distal end of the guide wire 18 exits thecatheter distal end 16 so that the catheter advances along the guidewire on a section of the catheter between the RX port 20 and thecatheter distal end 16. As is known in the art, the guide wire lumenwhich receives the guide wire is sized for receiving various diameterguide wires to suit a particular application. The stent is mounted onthe expandable member 22 (balloon) and is crimped tightly thereon sothat the stent and expandable member present a low profile diameter fordelivery through the arteries.

As shown in FIG. 1, a partial cross-section of an artery 24 is shownwith a small amount of plaque that has been previously treated by anangioplasty or other repair procedure. Stent 10 is used to repair adiseased or damaged arterial wall which may include the plaque 26 asshown in FIG. 1, or a dissection, or a flap which are sometimes found inthe coronary arteries, peripheral arteries and other vessels.

In a typical procedure to implant prior art stent 10, the guide wire 18is advanced through the patient's vascular system by well known methodsso that the distal end of the guide wire is advanced past the plaque ordiseased area 26. Prior to implanting the stent, the cardiologist maywish to perform an angioplasty procedure or other procedure (i.e.,atherectomy) in order to open the vessel and remodel the diseased area.Thereafter, the stent delivery catheter assembly 12 is advanced over theguide wire so that the stent is positioned in the target area. Theexpandable member or balloon 22 is inflated by well known means so thatit expands radially outwardly and in turn expands the stent radiallyoutwardly until the stent is apposed to the vessel wall. The expandablemember is then deflated and the catheter withdrawn from the patient'svascular system. The guide wire typically is left in the lumen forpost-dilatation procedures, if any, and subsequently is withdrawn fromthe patient's vascular system. As depicted in FIGS. 2 and 3, the balloonis fully inflated with the prior art stent expanded and pressed againstthe vessel wall, and in FIG. 3, the implanted stent remains in thevessel after the balloon has been deflated and the catheter assembly andguide wire have been withdrawn from the patient.

The prior art stent 10 serves to hold open the artery after the catheteris withdrawn, as illustrated by FIG. 3. Due to the formation of thestent from an elongated tubular member, the undulating components of thestent are relatively flat in transverse cross-section, so that when thestent is expanded, it is pressed into the wall of the artery and as aresult does not interfere with the blood flow through the artery. Thestent is pressed into the wall of the artery and will eventually becovered with endothelial cell growth which further minimizes blood flowinterference. The undulating portion of the stent provides good tackingcharacteristics to prevent stent movement within the artery.Furthermore, the closely spaced cylindrical elements at regularintervals provide uniform support for the wall of the artery, andconsequently are well adapted to tack up and hold in place small flapsor dissections in the wall of the artery, as illustrated in FIGS. 2 and3.

One of the problems associated with some prior art stents such as theone shown in FIG. 4, is the ability to more tightly crimp or compressthe stent 10 onto the balloon portion of the catheter. For example, theundulating portion 27 of the links 28 of the prior art stent in FIG. 4are positioned between two struts 29A/29B so that as the stent istightly crimped or compressed onto the balloon portion of the catheter,the struts can only come so close to the undulating portion beforecontact is made. Preferably, the undulating portion and the adjacentstruts should not overlap, therefore the undulating portion of the linklimits the amount of the crimping or compression of each cylindricalring onto the balloon portion of the catheter. The present inventionsolves this problem and allows for a tightly compressed or crimped stentonto the catheter.

In keeping with the present invention, FIGS. 6-16 depict stent 30 invarious configurations. Referring to FIG. 6A, for example stent 30 isshown in a flattened condition so that the pattern can be clearlyviewed, even though the stent is in a cylindrical form in use, such asshown in FIG. 6C. The stent is typically formed from a tubular member,however, it can be formed from a flat sheet such as shown in FIG. 6A androlled into a cylindrical configuration as shown in FIG. 6C.

As shown in FIGS. 6-16, stent 30 is made up of a plurality ofcylindrical rings 40 which extend circumferentially around the stentwhen it is in a tubular form (see FIGS. 6C, 7C, 8B, 9B, 10B, 11B and12). The stent has a delivery diameter 42 as shown in FIG. 12, and animplanted diameter 44 as shown in FIG. 13. Each cylindrical ring 40 hasa cylindrical ring proximal end 46 and a cylindrical ring distal end 48.Typically, since the stent is laser cut from a tube there are nodiscreet parts such as the described cylindrical rings and links.However, it is beneficial for identification and reference to variousparts to refer to the cylindrical rings and links and other parts of thestent as follows.

Each cylindrical ring 40 defines a cylindrical plane 50 which is a planedefined by the proximal and distal ends 46,48 of the ring and thecircumferential extent as the cylindrical ring travels around thecylinder. Each cylindrical ring includes cylindrical outer wall surface52 which defines the outermost surface of the stent, and cylindricalinner wall surface 53 which defines the innermost surface of the stent.Cylindrical plane 50 follows the cylindrical outer wall surface.

In keeping with the invention, undulating link 54 is positioned withincylindrical plane 50. The undulating links connect one cylindrical ring30 to an adjacent cylindrical ring 30 and contribute to the overalllongitudinal flexibility to the stent due to their unique construction.The flexibility of the undulating links derives in part from curvedportion 56 connected to straight portions 58 wherein the straightportions are substantially perpendicular to the longitudinal axis of thestent. Thus, as the stent is being delivered through a tortuous vessel,such as a coronary artery, the curved portions 56 and straight portions58 of the undulating links will permit the stent to flex in thelongitudinal direction which substantially enhances delivery of thestent to the target site. The number of bends and straight portions in alink can be increased or decreased from that shown, to achieve differingflexibility constructions. With the straight portions beingsubstantially perpendicular to the stent longitudinal axis, theundulating link acts much like a hinge at the curved portion to provideflexibility. A straight link that is parallel to the stent axistypically is not flexible and does not add to the flexibility of thestent.

Referring to FIGS. 6-16, the stent 30 can be described more particularlyas having a plurality of first peaks 60, second peaks 61, and valleys62. Although the stent is not divided into separate elements, for easeof discussion references to peaks and valleys is appropriate. The numberof peaks and valleys can vary in number for each ring depending upon theapplication. Thus, for example, if the stent is to be implanted in acoronary artery, a lesser number of peaks and valleys are required thanif the stent is implanted in a peripheral artery, which has a largerdiameter than a coronary artery. As can be seen for example in FIG. 6A,peaks 60,61 are in phase 63, meaning that the peaks 60,61 point in thesame direction and are substantially aligned along the longitudinal axisof the stent. It may be desirable under certain circumstances toposition the peaks so that they are out of phase (not shown), that is,the peaks of one ring would be circumferentially offset from the peaksof an adjacent ring so that the apex of adjacent peaks pointed towardeach other. As shown in FIGS. 6-16, the peaks are circumferentiallyoffset 64 from the valleys and from the undulating link 54. Positioningthe peaks, valleys, and undulating links in this manner, provides astent having uniform expansion capabilities, high radial strength, ahigh degree of flexibility, and sufficient wall coverage to support thevessel.

In keeping with the invention, and as shown in FIGS. 6-16, each of thecylindrical rings has a plurality of first peaks 60 which have firststruts 66 attached to a first apex 67. The first struts can be eithercurved or straight depending upon the particular application. Thecylindrical rings also have second peaks 61 which have second struts 68attached to a second apex 69. Again, the second struts can be eithercurved or straight depending upon the particular application.Importantly, the length of the second struts 68 are shorter than thelength of the first struts 66. As can be seen in FIGS. 6C, 7C, 8B, 9A,9B, 9C, 10A, 10B, 10C, 11A, 11B, 11C and 12, when the stent is in acrimped condition, or a partially crimped condition, the first strutsand second struts respectively will be closer to each other when thestent is compressed or crimped onto the balloon or expandable member ofthe catheter. The crimping or compressing process, however, also movesthe undulating link 54 along with its curved portion 56 closer to thesecond peak. In order to allow the stent to be more tightly crimped ontothe balloon portion of the catheter, and to avoid overlapping betweenthe undulating link and the second peak, the second struts 68 areshorter than the first struts 66, thus avoiding any overlapping contactbetween the curved portion of the undulating link and the second peak.The various stent struts, curved portions, links, and peaks and valleysmay contact each other when the stent is crimped or compressed, butoverlapping is an undesirable feature.

More particularly, in order to more tightly crimp or compress thecylindrical rings 40 of the stent 30, the undulating link 54 is tightlycrimped or compressed into contact with, or near contact with, secondpeak 61. As can be seen, for example, in FIG. 6C, curved portion 56 andstraight portions 58 are in close relation to second peak 61 and areeither in contact (not shown) or near contact with second apex 69. Thecurved portion is proximal to the second peak and the various struts ineach of the rings are tightly compressed to be in contact or nearcontact with each other. For example, first struts 56 and second struts58 as well as arm 76 of the undulating link all are in close contact, orcontact with each other in order to provide a very low profile, tightlycrimped stent onto the balloon portion of the catheter. Likewise, if thestent is formed of a self-expanding material such as nickel-titanium,the stent will similarly be tightly crimped and positioned within asheath or within the catheter for delivery in the vascular system.Importantly, the curved portion and the straight portions of theundulating link are positioned relative to the second peak to allow thestent to be tightly crimped as described.

As can be seen in FIGS. 6-16, there are slight variations in differingembodiments of the present invention. For example, the first struts 66and the second struts 68 of the stent depicted in FIGS. 6A-6C, arecurved and have several bends along their length. In contrast, as shownin FIGS. 9A-9C, the first struts and second struts are substantiallystraight. Whether the various struts are substantially straight or haveslight bends is a matter of choice to suit a particular application.

Referring to FIGS. 6-16, the stent 30 of the invention also can bedescribed as having cylindrical rings formed of U-shaped portions 70,Y-shaped portions 72, and W-shaped portions 74. Again, while the stentis generally laser cut from a tube and it typically has no discreetparts, for ease of identification the stent of the invention also can bereferred to as having U-, Y-, and W-shaped portions. The U-shapedportions have no supporting structure attached thereto. The Y-shapedportions, at their base, or apex, have arm 76 extending therefrom whichis attached to undulating link 54. The W portion has at its base orcurve portion an arm 78 which attaches at the other end of theundulating link. The length of the arms attaching the links to the ringscan vary.

Due to the intricate patterns as disclosed in FIGS. 6-13, the rate ofexpansion of the various portions of the stent, including the U-shapedportion 70, the Y-shaped portion 72, and the W-shaped portion 74, canvary. Accordingly, one aspect of the invention provides for differentradii of curvature at various points so that the stent will expandevenly and uniformly. Thus, first radius 71 which corresponds with firstpeak 60 has a smaller radius of curvature than does second radius 72which corresponds with second peak 61. Generally, the longer the strutsassociated with a peak, the more easily that portion of the stent willexpand, so that a smaller radius is associated with peaks having longerstruts. Likewise, for peaks, such as second peak 61, which has struts 68that are shorter than the struts 66 of first peak 60, has a greaterradius of curvature which will expand more easily in order to compensatefor the stiffer bending moments created by the shorter struts 68.

Also referring to FIGS. 6-13, the radius of curvature of the variousportions of the W-shaped portion also varies to provide uniform stentexpansion. Since the second peak 61 and its associated struts 68 have atendency to expand more slowly as the stent is expanded, a greaterradius of a curvature is provided in the adjacent part of the W-shapedportion 74. Thus, third radius 75 of the W-shaped portion 74 is greaterthan the fourth radius 77 in the W-shaped portion. The third radius 75is adjacent to second peak 61 which has a tendency to expand moreslowly, while fourth radius 77 is adjacent the first peak 60 which has atendency to expand more easily. By varying the radii of curvature in theW-shaped portion, the stent will expand more evenly and compensate forthe varying rates of expansion of adjacent portions in a cylindricalring.

It is also a design feature that more or fewer undulating links 54 willbe positioned between adjacent cylindrical rings 40. Further, in orderto increase stent stability, straight links 80, as shown in FIG. 11A, inaddition to undulating links 54, connect adjacent cylindrical rings. Thestraight links will provide stability and assist in preventing stentforeshortening, as do the undulating links. Further, the straight linksmay provide more rigidity in a localized area, such as at the stentends, such that it may be desirable to incorporate more straight linksbetween the cylindrical rings at the stent ends than in the center ofthe stent.

In an alternative embodiment as shown in FIG. 14, stent 30 is designedto provide good vessel wall coverage and greater expandability sinceeach cylindrical ring 40 has eight peaks 90. Generally, the more peaksin a cylindrical ring that has an undulating pattern, the greater theexpansion capabilities of that particular ring. Further, the stent ofFIG. 14 has a greater number of links 54 than in some of the other stentpatterns. In this embodiment, there are four undulating links 54 betweenadjacent rings so that the stent has uniform flexibility and maintainssufficient vessel wall coverage.

Referring to FIG. 15, an alternative embodiment of stent 30 is shown inwhich each cylindrical ring 40 has nine peaks 90. As with the stentpattern depicted in FIG. 14, the stent pattern of FIG. 15 is capable ofexpanding to a greater diameter due to the greater number of peaks 90and yet maintain sufficient vessel wall coverage. In this embodiment,the first peak 60 and second peak 61 are substantially the same aspreviously described with respect to the stent patterns depicted inFIGS. 6-13. In this embodiment, however, a third peak 92 has a pair ofthird struts 93 and a third apex 94. Third peak 92 has third struts 93that are longer than the first struts 66 and the second struts 68 of thefirst peak 60 and the second peak 61 respectively. As with the otherembodiments, the struts 66 of the first peak 60 are longer than thesecond struts 68 of second peak 61. Further, in order to provide moreuniform expansion of the stent, the third radius 95 of the third peak 92is smaller than the first radius 71 of first peak 60. Likewise, aspreviously described, first radius 71 is smaller than second radius 73of second peak 61. Generally speaking, the radius of curvature of thepeaks are inversely proportional to the length of the struts so that thelonger the struts the smaller the radius of curvature relative toshorter struts with a greater radius of curvature. As the stent expands,the peak having a greater radius of curvature will expand more easilythan those having a smaller radius of curvature, thus, compensating forthe length of the struts in which the peaks having shorter struts have atendency to expand more slowly than peaks having longer struts and whichhave moment arms that bend more easily.

Referring to FIG. 16, the stent 30 is similar to the other embodimentsexcept that the radius of curvature of all of the peaks and valleys aresomewhat larger in order to make it easier to laser cut the stentpattern from a tubular member or from a flat sheet.

Turning to FIG. 17, in an alternative embodiment, the stent 30 includesa pattern that does not have a so-called W-shaped portion. In thisembodiment, the undulating link 54 is substantially proximal to thesecond peak 61, with a slight portion of the undulating link 54 beingcircumferentially adjacent to the second peak. The first peak 60 stillhas struts 66 that are longer than struts 68 of second peak 61 so thatthe stent of this embodiment functions in substantially the same manneras that described for the other stent embodiments.

In one aspect of the invention, after stent 30 is implanted in acoronary artery, or other vessel, because of its novel design, thecylindrical rings 40 have the ability to flex radially as the vesselpulsates when blood pumps through it. Likewise, because of the novel andunique design of undulating links 54, as the vessel moves and pulsatesfrom the pumping blood, the stent can flex longitudinally. The radialand longitudinal flexing of the stent reduces the likelihood that thestent will cause injury to the intima of a coronary artery, which alsomay have a tendency to reduce the likelihood of restenosis.

In another aspect of the invention, the stent 30 is formed so that thevarious struts of the cylindrical rings, including the U-shaped portions70, Y-shaped portions 72, W-shaped portions 74, and the undulating links54, all can be formed so that each has a variable thickness along thestent length. For example, the undulating link, and its associated arms76,78 may be thicker at one end (arm 76) than at the other end of thelink (arm 78). Further, first struts 66 and second struts 68 may vary inthickness (radial thickness) along their length in order to createvariable flexibility in the rings. As shown in FIG. 16, first peak 60has first struts 66 that have radial thick portion 80 in the middle ofthe struts and radial thin portion 82 near the ends of the struts. Asanother example, the rings at for example the proximal end of the stentmay be thicker radially than the rings in the center of the stent. Avariable thickness stent that would benefit from the present inventionis described and disclosed in U.S. Ser. No. 09/343,962 filed Jun. 30,1999 and entitled VARIABLE THICKNESS STENT AND METHOD OF MANUFACTURETHEREOF, which is incorporated herein in its entirety by referencethereto. A variable thickness stent would benefit from the flexiblenature of the present invention stent and still be crimped to a very lowprofile delivery diameter due to the novel relationship between thesecond peak 61 and the undulating link 54.

The stent 30 of the present invention can be mounted on a ballooncatheter similar to that shown in the prior art device in FIG. 1. Thestent is tightly compressed or crimped onto the balloon portion of thecatheter and remains tightly crimped onto the balloon during deliverythrough the patient's vascular system. When the balloon is expanded, thestent expands radially outwardly into contact with the body lumen, forexample, a coronary artery. When the balloon portion of the catheter isdeflated, the catheter system is withdrawn from the patient and thestent remains implanted in the artery. Similarly, if the stent of thepresent invention is made from a self-expanding metal alloy, such asnickel-titanium or the like, the stent may be compressed or crimped ontoa catheter and a sheath (not shown) is placed over the stent to hold itin place until the stent is ready to be implanted in the patient. Suchsheaths are well known in the art. Further, such a self-expanding stentmay be compressed or crimped to a delivery diameter and placed within acatheter. Once the stent has been positioned within the artery, it ispushed out of the catheter or the catheter is withdrawn proximally andthe stent held in place until it exits the catheter and self-expandsinto contact with the wall of the artery. Balloon catheters andcatheters for delivering self-expanding stents are well known in theart.

The stent 30 of the present invention can be made in many ways. Onemethod of making the stent is to cut a thin-walled tubular member, suchas stainless steel tubing to remove portions of the tubing in thedesired pattern for the stent, leaving relatively untouched the portionsof the metallic tubing which are to form the stent. The stent also canbe made from other metal alloys such as tantalum, nickel-titanium,cobalt-chromium, titanium, shape memory and superelastic alloys, and thenobel metals such as gold or platinum. In accordance with the invention,it is preferred to cut the tubing in the desired pattern by means of amachine-controlled laser as is well known in the art.

The tubing may be made of suitable biocompatible material such asstainless steel. The stainless steel tube may be Alloy type: 316L SS,Special Chemistry per ASTM F138-92 or ASTM F139-92 grade 2. SpecialChemistry of type 316L per ASTM F138-92 or ASTM F139-92 Stainless Steelfor Surgical Implants in weight percent.

Carbon (C) 0.03% max. Manganese (Mn) 2.00% max. Phosphorous (P) 0.025%max.  Sulphur (S) 0.010% max.  Silicon (Si) 0.75% max. Chromium (Cr)17.00-19.00% Nickel (Ni) 13.00-15.50% Molybdenum (Mo) 2.00-3.00%Nitrogen (N) 0.10% max. Copper (Cu) 0.50% max. Iron (Fe) BalanceThe stent diameter is very small, so the tubing from which it is mademust necessarily also have a small diameter. Typically the stent has anouter diameter on the order of about 0.06 inch in the unexpandedcondition, the same outer diameter of the tubing from which it is made,and can be expanded to an outer diameter of 0.1 inch or more. The wallthickness of the tubing is about 0.003 inch.

The tubing is mounted in a rotatable collet fixture of amachine-controlled apparatus for positioning the tubing relative to alaser. According to machine-encoded instructions, the tubing is rotatedand moved longitudinally relative to the laser which is also machinecontrolled. The laser selectively removes the material from the tubingby ablation and a pattern is cut into the tube. The tube is thereforecut into the discrete pattern of the finished stent.

The process of cutting a pattern for the stent into the tubing isautomated except for loading and unloading the length of tubing. In oneexample, a CNC-opposing collet fixture for axial rotation of the lengthof tubing is used in conjunction with a CNC X/Y table to move the lengthof tubing axially relatively to a machine-controlled laser. The entirespace between collets can be patterned using the CO₂ laser set-up of theforegoing example. The program for control of the apparatus is dependenton the particular configuration used and the pattern to be ablated inthe coating.

Cutting a fine structure (0.005 to 0.001 inch web width) requiresminimal heat input and the ability to manipulate the tube withprecision. It is also necessary to support the tube yet not allow thestent structure to distort during the cutting operation. In order tosuccessfully achieve the desired end results, the entire system must beconfigured very carefully. The tubes are made typically of stainlesssteel with an outside diameter in the range of about 0.060 inch to 0.070inch and a wall thickness in the range of about 0.002 inch to 0.005inch. These tubes are fixtured under a laser and positioned utilizing aCNC to generate a very intricate and precise pattern. Due to the thinwall and the small geometry of the stent pattern (about 0.0035 inchtypical web width), it is necessary to have very precise control of thelaser, its power level, the focused spot size, and the precisepositioning of the laser cutting path.

In order to minimize the heat input into the stent structure, whichprevents thermal distortion, uncontrolled burn out of the metal, andmetallurgical damage due to excessive heat, and thereby produce a smoothdebris free cut, a Q-switched Nd-YAG, typically available fromQuantronix of Hauppauge, N.Y., that is frequency doubled to produce agreen beam at 532 nanometers is utilized. Q-switching produces veryshort pulses (<100 nS) of high peak powers (kilowatts), low energy perpulse (≦3 mJ), at high pulse rates (up to 40 kHz). The frequencydoubling of the beam from 1.06 microns to 0.532 microns allows the beamto be focused to a spot size that is 2 times smaller, thereforeincreasing the power density by a factor of 4 times. With all of theseparameters, it is possible to make smooth, narrow cuts in the stainlesstubes in very fine geometries without damaging the narrow struts thatmake up to stent structure. Hence, the system of the present inventionmakes it possible to adjust the laser parameters to cut narrow kerfwidth which will minimize the heat input into the material.

The positioning of the tubular structure requires the use of precisionCNC equipment such as that manufactured and sold by Anorad Corporation.In addition, a unique rotary mechanism has been provided that allows thecomputer program to be written as if the pattern were being cut from aflat sheet. This allows both circular and linear interpolation to beutilized in programming. Since the finished structure of the stent isvery small, a precision drive mechanism is required that supports anddrives both ends of the tubular structure as it is cut. Since both endsare driven, they must be aligned and precisely synchronized, otherwisethe stent structure would twist and distort as it is being cut.

The optical system which expands the original laser beam, delivers thebeam through a viewing head and focuses the beam onto the surface of thetube, incorporates a coaxial gas jet and nozzle that helps to removedebris from the kerf and cools the region where the beam interacts withthe material as the beam cuts and vaporizes the metal. It is alsonecessary to block the beam as it cuts through the top surface of thetube and prevent the beam, along with the molten metal and debris fromthe cut, from impinging on the opposite surface of the tube.

In addition to the laser and the CNC positioning equipment, the opticaldelivery system includes a beam expander to increase the laser beamdiameter, a circular polarizer, typically in the form of a quarter waveplate, to eliminate polarization effects in metal cutting, provisionsfor a spatial filter, a binocular viewing head and focusing lens, and acoaxial gas jet that provides for the introduction of a gas stream thatsurrounds the focused beam and is directed along the beam axis. Thecoaxial gas jet nozzle (0.018 inch I.D.) is centered around the focusedbeam with approximately 0.010 inch between the tip of the nozzle and thetubing. The jet is pressurized with oxygen at 20 psi and is directed atthe tube with the focused laser beam exiting the tip of the nozzle(0.018 inch dia.). The oxygen reacts with the metal to assist in thecutting process very similar to oxyacetylene cutting. The focused laserbeam acts as an ignition source and controls the reaction of the oxygenwith the metal. In this manner, it is possible to cut the material witha very fine kerf with precision. In order to prevent burning by the beamand/or molten slag on the far wall of the tube I.D., a stainless steelmandrel (approx. 0.034 inch dia.) is placed inside the tube and isallowed to roll on the bottom of the tube as the pattern is cut. Thisacts as a beam/debris block protecting the far wall I.D.

Alternatively, this may be accomplished by inserting a second tubeinside the stent tube which has an opening to trap the excess energy inthe beam which is transmitted through the kerf along which collectingthe debris that is ejected from the laser cut kerf. A vacuum or positivepressure can be placed in this shielding tube to remove the collectionof debris.

Another technique that could be utilized to remove the debris from thekerf and cool the surrounding material would be to use the inner beamblocking tube as an internal gas jet. By sealing one end of the tube andmaking a small hole in the side and placing it directly under thefocused laser beam, gas pressure could be applied creating a small jetthat would force the debris out of the laser cut kerf from the insideout. This would eliminate any debris from forming or collecting on theinside of the stent structure. It would place all the debris on theoutside. With the use of special protective coatings, the resultantdebris can be easily removed.

In most cases, the gas utilized in the jets may be reactive ornon-reactive (inert). In the case of reactive gas, oxygen or compressedair is used. Compressed air is used in this application since it offersmore control of the material removed and reduces the thermal effects ofthe material itself. Inert gas such as argon, helium, or nitrogen can beused to eliminate any oxidation of the cut material. The result is a cutedge with no oxidation, but there is usually a tail of molten materialthat collects along the exit side of the gas jet that must bemechanically or chemically removed after the cutting operation.

The cutting process utilizing oxygen with the finely focused green beamresults in a very narrow kerf (approx. 0.0005 inch) with the molten slagre-solidifying along the cut. This traps the cut out scrap of thepattern requiring further processing. In order to remove the slag debrisfrom the cut allowing the scrap to be removed from the remaining stentpattern, it is necessary to soak the cut tube in a solution of HCl forapproximately 8 minutes at a temperature of approximately 55° C. Beforeit is soaked, the tube is placed in a bath of alcohol/water solution andultrasonically cleaned for approximately 1 minute to remove the loosedebris left from the cutting operation. After soaking, the tube is thenultrasonically cleaned in the heated HCl for 1-4 minutes depending uponthe wall thickness. To prevent cracking/breaking of the struts attachedto the material left at the two ends of the stent pattern due toharmonic oscillations induced by the ultrasonic cleaner, a mandrel isplaced down the center of the tube during the cleaning/scrap removalprocess. At completion of this process, the stent structure are rinsedin water. They are now ready for electropolishing.

The stents are preferably electrochemically polished in an acidicaqueous solution such as a solution of ELECTRO-GLO#300, sold byELECTRO-GLO Co., Inc. in Chicago, Ill., which is a mixture of sulfuricacid, carboxylic acids, phosphates, corrosion inhibitors and abiodegradable surface active agent. The bath temperature is maintainedat about 110°-135° F. and the current density is about 0.4 to about 1.5amps per in.². Cathode to anode area should be at least about two toone. The stents may be further treated if desired, for example byapplying a biocompatible coating.

It will be apparent that both focused laser spot size and depth of focuscan be controlled by selecting beam diameter and focal length for thefocusing lens. It will be apparent that increasing laser beam diameter,or reducing lens focal length, reduces spot size at the cost of depth offield.

Direct laser cutting produces edges which are essentially perpendicularto the axis of the laser cutting beam, in contrast with chemical etchingand the like which produce pattern edges which are angled. Hence, thelaser cutting process essentially provides strut cross-sections, fromcut-to-cut, which are square or rectangular, rather than trapezoidal.The struts have generally perpendicular edges formed by the laser cut.The resulting stent structure provides superior performance.

Other methods of forming the stent of the present invention can be used,such as using different types of lasers; chemical etching; electricdischarge machining; laser cutting a flat sheet and rolling it into acylinder; and the like, all of which are well known in the art at thistime.

The stent of the present invention also can be made from metal alloysother than stainless steel, such as shape memory alloys. Shape memoryalloys are well known and include, but are not limited to,nickel-titanium and nickel-titanium-vanadium. Any of the shape memoryalloys can be formed into a tube and laser cut in order to form thepattern of the stent of the present invention. As is well known, theshape memory alloys of the stent of the present invention can includethe type having superelastic or thermoelastic martensitictransformation, or display stress-induced martensite. These types ofalloys are well known in the art and need not be further described here.

Importantly, a stent formed of shape memory alloys, whether thethermoelastic or the stress-induced martensite-type, can be deliveredusing a balloon catheter of the type described herein, or be deliveredvia a catheter without a balloon or a sheath catheter.

While the invention has been illustrated and described herein, in termsof its use as an intravascular stent, it will be apparent to thoseskilled in the art that the stent can be used in other body lumens.Further, particular sizes and dimensions, number of undulations orU-shaped portions per ring, materials used, and the like have beendescribed herein and are provided as examples only. Other modificationsand improvements may be made without departing from the scope of theinvention.

What is claimed is:
 1. A flexible intravascular stent for use in a bodylumen, comprising: a plurality of cylindrical rings aligned along acommon longitudinal axis and interconnected to form the stent, eachcylindrical ring having a first delivery diameter and a second implanteddiameter; each cylindrical ring having a plurality of first peaks, eachof the first peaks having a first height; each cylindrical ring having aplurality of second peaks, each second peak having a second height, thesecond height being shorter than the first height; each cylindrical ringhaving a plurality of valleys; and a plurality of undulating linksattaching each cylindrical ring to an adjacent cylindrical ring, theundulating links having a curved portion extending transverse to thestent longitudinal axis toward the second peak, the second height of thesecond peak being sized so that as the stent is compressed to the firstdelivery diameter, the curved portion of the undulating link islongitudinally aligned with the second peak, wherein each undulatinglink has a first end attached to a valley of a cylindrical ring and asecond end attached to a valley of an adjacent cylindrical ring, the twoadjacent valleys which are attached to each undulating link being inphase with each other when the cylindrical rings are in the firstdelivery diameter, the undulating links attaching a first set ofcylindrical rings being circumferentially offset 60° from the undulatinglinks attaching an adjacent second set of cylindrical rings; and theundulating links attaching every other set of cylindrical rings beinglongitudinally aligned along a longitudinal axis of the stent.
 2. Thestent of claim 1, further including a proximal end ring and a distal endring, each of the distal end ring and the proximal end ring beingconnected to adjacent cylindrical rings by three straight links.
 3. Thestent of claim 2, the straight links are less flexible longitudinallythan are the undulating links.
 4. A flexible intravascular stent for usein a body lumen, comprising: a plurality of cylindrical rings alignedalong a common longitudinal axis and interconnected to form the stent,each cylindrical ring having a first delivery diameter and a secondimplanted diameter; each cylindrical ring having a plurality of firstpeaks, each of the first peaks having a first height; each cylindricalring having a plurality of second peaks, each second peak having asecond height, the second height being shorter than the first height;each cylindrical ring having a plurality of valleys; and an undulatinglink attaching each cylindrical ring to an adjacent cylindrical ring,the undulating link having a curved portion extending transverse to thestent longitudinal axis toward the second peak, the second height of thesecond peak being sized so that as the stent is compressed to the firstdelivery diameter, the curved portion of the undulating link islongitudinally aligned with the second peak, wherein the undulating linkhas a first end attached to a valley of a cylindrical ring and a secondend attached to a valley of an adjacent cylindrical ring, the twoadjacent valleys which are attached to the undulating link being inphase with each other when the cylindrical rings are in the firstdelivery diameter, the undulating links attaching a first set ofcylindrical rings being circumferentially offset 60° from the undulatinglinks attaching an adjacent second set of cylindrical rings; and theundulating links attaching every other set of cylindrical rings beinglongitudinally aligned along a longitudinal axis of the stent.
 5. Thestent of claim 4, further including a proximal end ring and a distal endring, wherein both the distal end ring and the proximal end ring areconnected to an adjacent cylindrical ring by a straight link.
 6. Thestent of claim 4, further including a proximal end ring and a distal endring, wherein each of the distal end ring and the proximal end ring isconnected to adjacent cylindrical rings by three straight links.
 7. Thestent of claim 5, wherein the straight links are less flexiblelongitudinally than are the undulating links.